<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			/*
			b自动居中显示
			*/
			.a{
				width: 200px;
				height: 200px;
				border: 1px solid;
			}
			.b{
				margin: 0 auto;
				height: 200px;
				width: 100px;
				background-color: red;
			}
		</style>
	</head>
	<body>
		<div class="a">
			<div class="b"></div>
		</div>
	</body>
</html>
